Jeremy RennerMy Puppy Tragically Died

3/14/2012 10:45 AM PDT

Jeremy Renner -- My Puppy Tragically Died

Jeremy Renner suffered a devastating loss last weekend, after his 8-month-old puppy Franklin dropped dead on a baseball field.

Jeremy's friend and business partner Kristoffer Winters was dogsitting the French bulldog last Saturday in Orange County -- and decided to take it to his nephew's little league baseball game.

We're told the dog was running around the field having fun on a very hot day -- near 80 degrees. Then, at around 2 PM, the pup began to vomit and convulse. Kris -- who had left to get a snack -- returned to find Jeremy's dog dead in a woman's arms.

We're told 3 people tried to give the dog mouth-to-mouth to revive the pooch -- to no avail.

French bulldogs notoriously suffer from heart problems, which could be exacerbated by heat.

We're told Jeremy and Franklin were EXTREMELY close and he's grief-stricken.